눈빛 is used to describe the feeling that one;s eyes give off. “눈빛이 빛나다,” means that someone;s “eyes are sparkling/twinkling/shiny” with delight. If someone says “눈빛이 무섭다,” it means that someone’s eyes are scary/fierce.” These phrases can be used literally, the the expression 눈빛만 봐도 알다 cannot be used in such a literal sense, as it refers to two people being so close/connected/intimte with one another that one person can tell by that the other person wants or is trying to say just by looking at the other’s eyes.

오늘은 효리씨에게 데이트 신청할거야. I am really going to ask Hyori out for dating.

짝사랑 그만하고 진짜 용기를 내 봐. Stop just having a crush and really summon up some courage.

짝사랑이 아니야. 효리씨도 나한테 관심이 있다고. It is not a crush. She is really interested in me too.

니가 어떻게 알아. How do you know that?

효연씨 눈빛만 봐도 다 알아. 나를 좋아하고 있다고. I look at her eyes and I know it. She likes me.